变量定义的规则：
- 变量名只能是字母、数字、或下划线的任意组合
- 变量名的第一个字符不能是数字
- 以下关键字不能声明为变量名
['and','as','assert','break','class','continue','def','del','else','except','exec',finally','for','from',
'global','if','import','in','is','lambda','not','or','pass','print','raise','return','try','while','with','yield']